1. Field of the Invention
This invention relates to a high-pressure fuel injection pipe having a connecting head portion at a free end thereof made of a thick-walled pipe having a relatively small diameter of about 4 m/m to about 20 m/m and a wall thickness of about 1 m/m to 8 m/m used widely in a suitably arranged state as a fuel supply path and the like for, for example, a diesel internal combustion engines.
2. Description of the Prior Art
The known high-pressure fuel injection pipes of this kind include high-pressure fuel injection pipe having a connecting head portion (JP-A-8-75075) having as shown as an example in FIG. 4 at a connecting end portion of a thick-walled steel pipe 11 having a relatively small diameter a connecting head portion 12 formed with a seat surface 13, an annular flange 15 formed so that the flange 15 is axially spaced from the seat surface 13, and a conical surface 14 connected to the seat surface 13, extending therefrom to the annular flange 15 and tapering off to a free end of the connecting head portion 12 This kind of connecting head portion 12 is molded by a buckling process carried out by axially pressing the same connecting head portion 12 from the outside by a punch member. Concerning this molding operation, the connecting head portion 12 is formed with a deep, sharp and large annular pocket 16 occurring in an inner circumferential surface thereof due to an outward expansion of a circumferential wall thereof accompanied by the buckling process based on the punch-pressing opera,ion. This high-pressure fuel injection pipe in such a condition is put to practical use at present.
However, such a prior art high-pressure fuel injection pipe having a connecting head portion has a problem of causing during a practical use of the injection pipe the cavitation erosion of a part thereof around a deep, sharp and large annular pocket, which is formed in an inner surface of a connecting head portion, to occur during a practical use thereof due to a high-pressure fluid, and a problem of having the possibility that a fatigue failure of the fuel injection pipe starts at the deepest part of the pocket portion.
The present invention has been made in view of these problems encountered in the above-mentioned prior art fuel injection pipe. One of the objects of the present invention is to provide a high-pressure fuel injection pipe having connecting head portion at free end thereof capable of having a contour of a cross section of an annular pocket occurring in an inner part of the connecting head portion during a head portion processing operation extend shallowly and gently, and therefore capable of eliminating a fear of the occurrence of cavitation erosion of an inner part of the connecting head portion and a tear of the occurrence of a fatigue failure of the same head portion